PURPOSE: To speedily and accurately measure the filtration characteristics of the sludge put in a sample pot by concreting all processes for dipping filter paper in the sludge to specific depth by using various air cylinders.

CONSTITUTION: Plural sheets of filter paper which are striped and nondirectional and have water absorptivity are stacked and stored in a paper filter holder 1 and a bottom plate 13 where filter paper 2 is mounted is elevated by the extension of the rod of an air cylinder 14 for filter paper elevation. Namely, a crank plate 15 is provided successively to one end part of the bottom plate 13, the rod of the cylinder 14 is fixed to the upper projection piece 16 of the crank plate 15, and the bottom plate 13 is put in elevation motion according to the extension and contraction of the rod. An optical sensor 17 for filter paper detection which consists of a photoelectric light emission body and a light reception body is provided additionally at the upper position of the holder 1 and the bottom plate 13 is elevated with the signal sent out of the optical sensor 17 to hold the upper end of the filter paper 2 at the same position in the holder 1 at all times.